Lost custom colours

[alasdair105]

Using WP 6.1 running twenty twenty three theme and no plugins.
I set custom colours, which then disappear when I do this:
Set up custom colour, save view the site, edit the site, custom colours still there.

However, if I go and edit the style json file (only a minor change - so no errors in json file), then I load the changed json style, then save, view site and edit site again ... the custom colours are not there.

(I don't know where custom colours are kept - so I've assumed this may be a bug, rather than by design?)

Thanks

Alasdair